Oral microbial biofilms coat the hard and soft tissues of the mouth. Although the composition of the biofilms is heterogeneous and varies considerably by site, populations at a given site remain fairly stable and are usually compatible with the maintenance of the integrity of the colonized tissue. However, changes in the environment can perturb the balance in biofilms, leading to shifts in the proportions of particular species and alterations in the spectrum of metabolic activities, which can lead to oral diseases. For example, sustained low pH favors the growth of mutans streptococci in caries development, and as yet uncharacterized changes in the periodontium leading to increases in the populations of proteolytic, Gram-negative anaerobic bacteria appear necessary for periodontal diseases. Among the environmental factors known i) to affect the expression of known virulence attributes of oral bacteria, and ii) to induce dramatic changes in the composition and activity of oral biofilms, pH, carbohydrate source and availability, and oxygen have perhaps the most profound influences. This proposal describes a collaborative, multidisciplinary research effort by three investigators that addresses central hypotheses about homeostasis in biofilms, gene expression by adherent bacteria and the role of the environment in the development of pathogenic biofilms. Coupling recombinant DNA techniques to construct defined mutants and gene fusions, with physiologic manipulations, cultivation of mono-species and defined consortia, and scanning confocal laser microscopy, the effects of, and responses to, environmental pH, carbohydrate availability, and oxygen by individual species in biofilms and by complex populations will be explored. The results obtained will provide much needed, fundamental knowledge about the phenotypic capacities of oral bacteria in biofilms and yield new insights into potential mechanisms for modulating plaque physiology and ecology to inhibit the development of oral diseases.